Stabilizing mutation of CTNNB1/beta-catenin and protein accumulation analyzed in a large series of parathyroid tumors of Swedish patients
© Björklund et al; licensee BioMed Central Ltd. 2008
Received: 26 February 2008
Accepted: 09 June 2008
Published: 09 June 2008
Aberrant accumulation of β-catenin plays an important role in a variety of human neoplasms. We recently reported accumulation of β-catenin in parathyroid adenomas from patients with primary hyperparathyroidism (pHPT). In CTNNB1 exon 3, we detected a stabilizing mutation (S37A) in 3 out of 20 analyzed adenomas. The aim of the present study was to determine the frequency and zygosity of mutations in CTNNB1 exon 3, and β-catenin accumulation in a large series of parathyroid adenomas of Swedish patients.
The mutation S37A (TCT > GCT) was detected by direct DNA sequencing of PCR fragments in 6 out of 104 sporadic parathyroid adenomas (5.8%). Taking our previous study into account, a total of 9 out of 124 (7.3%) adenomas displayed the same mutation. The mutations were homozygous by DNA sequencing, restriction enzyme cleavage, and gene copy number determination using the GeneChip 500 K Mapping Array Set. All tumors analyzed by immunohistochemistry, including those with mutation, displayed aberrant β-catenin accumulation. Western blotting revealed a slightly higher expression level of β-catenin and nonphosphorylated active β-catenin in tumors with mutation compared to those without. Presence of the mutation was not related to distinct clinical characteristics.
Aberrant accumulation of β-catenin is very common in parathyroid tumors, and is caused by stabilizing homozygous mutation in 7.3% of Swedish pHPT patients.
Parathyroid disease with hypersecretion of parathyroid hormone and generally also hypercalcemia occurs in primary hyperparathyroidism (pHPT), due to growth regulatory disturbance in one or several parathyroid glands. Activation of CCND1 oncogene expression or inactivation of the MEN1 tumor suppressor gene contributes to deregulated growth control in a fraction of sporadic parathyroid adenomas [1–4].
Activation of the Wnt/β-catenin signaling pathway by aberrant accumulation of stabilized β-catenin is involved in the development of many neoplasms. β-catenin accumulation is typically caused by mutations in components of the signaling pathway, such as APC, Axin, β-Trcp, and WTX, or results from secondary events. In addition, protein stabilizing mutations in the glycogen synthase kinase 3β phosphorylation sites of β-catenin (Ser-33, Ser-37, Thr-41, Ser-45) occur with varying frequency in several neoplasms [5–9].
We recently reported activation of the Wnt/β-catenin signaling pathway by aberrant accumulation of β-catenin in parathyroid adenomas from patients with pHPT . The accumulation of β-catenin was caused by expression of an aberrantly spliced internally truncated Wnt receptor LRP5 or by a stabilizing mutation (S37A) in CTNNB1 exon 3 [10, 11]. Stabilizing mutations of CTNNB1 have not been detected in parathyroid adenomas of patients from Japan and the United States [12, 13]. Here we have determined the frequency and zygosity of mutations in exon 3 of CTNNB1, and β-catenin expression status in a large series of parathyroid adenomas of Swedish patients.
Sporadic parathyroid adenomas (n = 104) were acquired from 104 Swedish patients with pHPT diagnosed and operated on in the clinical routine at the Uppsala University Hospital. Normal parathyroid tissue was obtained as normal gland biopsies in patients subjected to parathyroidectomy. Tissues were intraoperatively snap frozen. Informed consent and approval of institutional ethical committee were obtained.
DNA from parathyroid tumors was prepared by standard procedures including proteinase K treatment and phenol extraction. Blood DNA was prepared using the Wizard Genomic DNA Purification Kit (Promega Corp., Madison, WI). DNA was PCR amplified with primers for exon 3 of CTNNB1. PCR forward primer: 5'-TGA TGG AGT TGG ACA TGG CC; reverse: 5'-CTC ATA CAG GAC TTG GGA GG. The complementary strand was also sequenced for fragments with mutation. The PCR fragments were sequenced directly on the 3130xl Genetic Analyzer using the ABI Prism Dye Terminator Cycle Sequencing Ready Reaction kit (Applied Biosystems, Foster City, CA).
Restriction Enzyme Digestion
CTNNB1 exon 3 PCR fragments were purified using the GFX PCR DNA and Gel Band Purification Kit (GE Healthcare Europe GmbH, Uppsala, Sweden) and cleaved with Xma I or Nla III according to instructions by the manufacturer (New England Biolabs, Inc., Beverly, MA). Products were analyzed by agarose gel electrophoresis.
CTNNB1 Gene Copy Number
Immunohistochemistry and Western Blotting
Frozen tissue sections were stained as described  using an anti-β-catenin goat polyclonal antibody with an epitope mapping at the C-terminus (Santa Cruz Biotechnology, INC., Santa Cruz, CA; catalog no. sc-1496). Protein extracts for Western blotting were prepared  in Cytobuster Protein Extract Reagent (Novagen Inc., Madison, Wisconsin, USA) supplemented with Complete protease inhibitor cocktail (Roche Diagnostics GmbH, Penzberg, Germany). The anti-active (nonphosphorylated) β-catenin  mouse monoclonal antibody (Upstate, Lake Placid, USA, # 05-665), the anti-β-catenin goat polyclonal antibody (above), and anti-actin goat polyclonal antibody (Santa Cruz Biotechnology INC.) were used. After incubation with the appropriate secondary antibodies, bands were visualized using the enhanced chemiluminescence system (GE Healthcare Europe GmbH, Uppsala, Sweden). Membranes were scanned by the ChemiDoc XRS and the band intensities were determined using Quantity One Software (Bio-Rad Laboratories, Inc., Hercules, California, USA).
Unpaired t test, z test, and χ2 test were used. The data were calculated with Statistica 6 (StatSoft, Tulsa, OK, USA). Values are presented as arithmetrical mean ± SEM.
Homozygous CTNNB1 Stabilizing Mutation S37A
In order to resolve the issue of zygosity for the S37A mutation, 4 tumor DNAs with the corresponding constitutional DNAs were genotyped with the GeneChip 500 K Mapping Array Set (Affymetrix). Gene copy number analysis was done by comparison of informative single-nucleotide polymorphisms (SNPs). Five informative SNPs in CTNNB1 and 2 SNPs downstream of the gene showed equal copy number for the 4 paired DNA samples (Table 1). Thus, taking also the DNA sequencing results into account the S37A mutation was homozygous in these 4 tumours, rather than hemizygous with one mutant and one deleted CTNNB1 allele.
β-catenin Protein Expression
We have found that a total of 9 out of 124 (7.3%) randomly selected parathyroid adenomas displayed the CTNNB1 stabilizing homozygous mutation S37A. None of these 9 tumors expressed the internally truncated LRP5 receptor  (data not shown), further emphasizing the previous observation that these events are mutually exclusive . The mutated LRP5 receptor, with the central region deleted, is expressed in the majority of pHPT tumors and is required for accumulation of β-catenin and parathyroid tumor cell growth .
The S37A CTNNB1 mutation commonly occurs also in gastrointestinal carcinoid tumors where 26 out of 29 tumors with mutations harboured S37A . The mutant protein shows resistance to ubiquination and proteosomal degradation, with a longer half-life than wild-type β-catenin. S37A β-catenin also shows an enhanced affinity for LEF1 and TCF4, its DNA-binding partners in transcriptional regulation [16–18]. Homozygous mutation as detected by direct DNA sequencing of CTNNB1 seems to be uncommon in other neoplasms, but have been described in a rectal carcinoid tumor and in colorectal cancer [15, 19]. To be conclusive regarding zygosity, direct DNA sequencing clearly requires low or no contamination of normal cell populations in the tumor sample. In colorectal cancer cells with inactivating APC hemizygous mutation or activating CTNNB1 heterozygous mutation, the total β-catenin signaling activity seemed dependent also on silencing of SFRP genes by promoter hypermethylation with consistent constitutive WNT signaling . Whether combined activity of two mutant S37A CTNNB1 alleles suffices for benign parathyroid tumor growth or whether constitutive WNT signaling is required in addition, remain to be investigated.
DNA sequence analysis of 24 parathyroid adenomas from Japanese patients revealed no CTNNB1 mutations, and immunohistochemistry showed weak cytoplasmic β-catenin staining in 2 tumors . In another study from Japan (n = 9), cytoplasmic and/or membranous β-catenin staining was seen in 8 adenomas and nuclear staining in one adenoma. DNA sequencing analysis was not done in these specimens . Furthermore, a recent study did not detect CTNNB1 exon 3 mutations in 97 sporadic parathyroid adenomas from patients who had undergone parathyroidectomy in the United States. Unfortunately, β-catenin protein expression was not evaluated in this report . Since we observed a CTNNB1 mutation frequency of 7.3% in adenomas from Swedish patients, this may suggest possible contribution of geographical origin (dietary or environmental differences, or different genetic backgrounds) to mechanisms of parathyroid disease. The CTNNB1 mutation frequency vary considerably also in colorectal cancer (1–60%) and melanomas (0.02–27%), apparently not related to geographical origin [8, 17, 22–30]. The observations may be attributed to the stochastic distribution of probability in analyzed material or to other causalities, like that the tumor sample purity and pathology must be guaranteed.
By analyzing a large series of tumors from Swedish patients, this study further emphasizes β-catenin accumulation as the most common aberration in parathyroid tumors of primary origin. CTNNB1-stabilizing mutations were found in 5.8% of the tumors, or in 7.3% when taking our previous study  into account. The WNT/β-catenin signaling pathway, with β-catenin and the internally truncated LRP5 receptor  in particular, present therapeutic targets for hyperparathyroidism.
This work was supported by the Swedish Research Council and Swedish Cancer Society. We thank the referee of another paper for suggesting the use of Xma I and Nla III. We are grateful to Birgitta Bondeson and Peter Lillhager for skilful technical assistance. We are also grateful to Marika Rönnholm at the Bioinformatics and Expression Analysis core facility at NOVUM, Karolinska Institute, Huddinge, Sweden.
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